http://news.discovery.com/space/meteor- ... 20423.html wrote:
Asteroid the 'Size of a Minivan' Exploded over California
by Ian O'Neill, Apr 23, 2012, Discovery News
<<The source of loud "booms" accompanied by a bright object traveling through the skies of Nevada and California on Sunday morning has been confirmed: it was a meteor. A big one.
It is thought to have been a small asteroid that slammed into the atmosphere at a speed of 15 kilometers per second, turning into a fireball, delivering an energy of 3.8 kilotons of TNT as it broke up over California's Sierra Nevada mountains. Bill Cooke, head of NASA's Meteoroid Environment Office, classified it as a "big event."
"I am not saying there was a 3.8 kiloton explosion on the ground in California," Cooke told Spaceweather.com. "I am saying that the meteor possessed this amount of energy before it broke apart in the atmosphere. (The map) shows the location of the atmospheric breakup, not impact with the ground."
Cooke went on to say that the meteor likely penetrated very deep into the atmosphere, producing the powerful sonic booms that rattled homes across the region. According to Reuters, car alarms in Carson City, Nev., were even triggered.
After some rough calculations, Cooke has been able to estimate the mass of the incoming object -- around 70 metric tons. This was a fairly hefty piece of space rock. From this estimate he was also able to arrive at an approximate size of the meteor: "Hazarding a further guess at the density of 3 grams per cubic centimeter (solid rock), I calculate a size of about 3-4 meters, or about the size of a minivan."
Although there were numerous reports of sightings in Nevada and California, there was little immediate clue as to where the fireball ended its journey. But with the help of two infrasound stations, the source of the explosion could be resolved.
"Elizabeth Silber at Western University has searched for infrasound signals from the explosion," said Cooke. "Infrasound is very low frequency sound which can travel great distances. There were strong signals at 2 stations, enabling a triangulation of the energy source at 37.6N, 120.5W. This is marked by a yellow flag in the map (above)."
Interestingly, the estimated size of the California fireball is bigger than the small 3-meter wide asteroid that exploded over Sudan in 2008. That one delivered an energy of 1.1–2.1 kilotons of TNT. Asteroid 2008 TC3 was actually the first ever space rock to be detected before it hit the Earth's atmosphere. With astonishing accuracy, astronomers at Mount Lemmon telescope in Arizona managed to spot the tiny asteroid and infrasound stations in Kenya triangulated the location where the asteroid hit. Using this information, meteorite hunters were able to recover fragments of the impact.
This time, however, we didn't spot the California-bound asteroid coming, but we know where it ended up. Perhaps it's a good time to dispatch meteorite hunters to the Sierra Mountains.>>